The present invention relates to a low dielectric loss tangent resin composition used for insulating materials of electrical parts having low dielectric loss for corresponding to high frequency signals, cured products of the composition and electrical pats using the same.
In recent years, the signal band of information communication apparatuses, such as PHS and portable telephones, and the CPU clock time of computers have reached the GHz band, and the trend toward higher frequency is increasingly strong.
The dielectric loss of an electric signal is proportional to the square root of dielectric constant of the insulating material constituting the circuit and to the product of the dielectric loss tangent and the frequency of the signal used. Therefore, the higher the frequency of the signal, the larger the dielectric loss. Since dielectric loss causes attenuation of electric signals to lower the reliability of the signals, it has been necessary to select as an insulator a material which has a low dielectric constant and low dielectric loss tangent.
For obtaining an insulating material having a low dielectric constant and low dielectric loss tangent, it is effective to remove polar groups in its structure, and there have been proposed fluororesin, curable polyolefin, cyanate ester resin, curable polyphenylene oxide, allyl-modified polyphenylene ether and polyether imide modified with divinylbenzene or divinylnaphthalene.
Fluororesins typically represented by polytetrafluoroethylene (PTFE) have both a low dielectric constant and a low dielectric loss tangent and are used as materials for substrates which deal with high frequency signals. However, since PTFE is a thermoplastic resin, it undergoes large expansion and shrinkage in molding and processing and hence is a material not easy to handle.
A number of proposals have been made to impart crosslinkability or solubility to fluororesin. However, materials thus obtained are generally expensive, and many of them are not comparable to fluororesin in characteristic properties.
On the other hand, various studies have been made to obtain non-fluorine type resins of low dielectric constant and low dielectric loss tangent which are soluble in organic solvents and are easy to handle.
For example, there have been disclosed a product obtained by impregnating glass cloth with a diene-type polymer, such as polybutadiene, followed by curing with a peroxide (JP-A-8-208856), a cyclic polyolefin obtained by introducing an epoxy group into a norbornene-type addition polymer thereby to impart curability to the product (JP-A-10-158337) and products obtained by heating cyanate ester, diene-type polymer and epoxy resin to reach the B-stage (JP-A-11-124,491).
There have been further disclosed many example, which include a modified resin comprising polyphenylene oxide, diene-type polymer and triallyl isocyanate (JP-A-9-118759), a resin composition comprising allyl-modified polyphenylene ether, triallyl isocyanate, etc. (JP-A-9-246429), an alloyed products of polyether imide with styrene and divinylbenzene and/or divinylnaphthalene (JP-A-5-156159), a product synthesized by the Williamson's reaction from a dihydroxy compound and chloromethylstyrene, e.g., a resin composition comprising hydroquinone bis(vinylbenzyl) ether and novolac phenol resin (JP-A-5-78552).
Many of the examples mentioned above include in their disclosures a description that the cited product may contain divinylbenzene as a crosslinking agent or a crosslinking auxiliary. This can be attributed to the fact that divinylbenzene has no polar group in its structure and the cured product thereof has a low dielectric constant and low dielectric loss tangent and has a heat decomposition temperature of as high as 350° C. or more.
However, divinylbenzene has a defect in that since the cured product thereof is very brittle, cracks tend to develop in the cured product at the time of curing. Therefore, the amount of divinylbenzene added has been usually set at a low level as compared with other resin components.
Even in the example disclosed in JP-A-5-156159 wherein divinylbenzene is used as the main crosslinking agent, the amount added is about 9% by weight relative to the whole of the resin. Divinylnaphthalene also has a problem similar to that of divinylbenzene in point of brittleness of the cured product. Furthermore, since divinylbenzene is volatile, it vaporizes at the time of curing, making it difficult to control the characteristic property of the cured product.
As contrasted therewith, JP-A-5-78552 discloses that such bisstyrene compounds as hydroquinone bis(vinylbenzyl) ether are nonvolatile and can give a highly flexible cured product.
In general, however, an alkylene ether group is disadvantageous with respect to dielectric constant, dielectric loss tangent and heat resistance as compared with an alkylene group and arylene group.
A skeleton of hydrocarbon type, e.g., an alkylene group and arylene group, is preferable as the skeleton structure linking between styrene groups. Examples of a multifunctional styrene compound wherein styrene groups are linked by an ethylene group include 1,2-bisvinylphenylethane described in JP-A-9-208625 and a divinylbenzene oligomer having a vinyl group in the side chain described in Makromol. Chem. Vol. 187, pp 23 (1986). In these reports, however, no investigation was made regarding mechanical strength, heat resistance, dielectric constant and dielectric loss tangent.
Divinylbenzene, which has hitherto been used as a low dielectric constant and low dielectric loss tangent crosslinking agent, is disadvantageous in that it is volatile and its cured product is brittle.